One of the major applications of conducting polymers is as a component of an “organic” or “bulk heterojunction” photovoltaic cell for making electricity from sunlight. Organic solar cells generally include a layer of transparent indium tin oxide (ITO) on glass, a layer of hole-conducting polymer mixed with an organic or inorganic electron-conducting phase, and a third layer of a low-work-function metal such as aluminum. Light is absorbed by the polymer in the photovoltaic cell; the resulting excitation then migrates to a phase boundary with the electron-conducting phase, and an electron is injected from the polymer. The electrons migrate to the metal through the electron-conducting phase, and holes migrate to the ITO via the polymer, generating a photocurrent.
Conducting polymers for use in organic light-emitting diodes (OLEDs) and organic conductors (“wires”), some of which may be electropolymerized, have been reported. These polymers include, for example, polyacetylenes, polyphenyleneethynylenes (PPE), polyphenylenevinylenes (PPV), polythiophenes, and polyanilines. The structural variations of these polymers are typically changes in the organic substituent groups attached to the basic polymer backbone. The substituents can change the electrical properties of the polymer, its processing and mechanical properties, and its compatibility/interfacing with other materials. Typically, it is possible to write delocalized structures for these polymers that allow conductivity through the polymer backbone. Often, some type of “doping” is required to achieve good conductivity.
Such organic photovoltaic cells have very low efficiencies (i.e., less than about 5%) due to incomplete conversion of excitons to charge separation, recombination of electrons and holes within the organic layer or at the electrodes, and a lack of efficient light absorption throughout the solar spectral range. The lack of electron acceptors covalently bonded to the polymer strands can lead to inefficient conversion of excitons to charge separation, rapid charge recombination, high concentrations of carriers at interface areas (leading to increased recombination), the need to prepare bicontinuous phases of polymer and fullerene or other electron carriers (which adds to processing requirements), and the need to use excessive amounts of the electron transport phase.